1. Field of the Invention
The present invention relates generally to data storage systems, and more particularly to a data storage system attached to a data network for receiving and storing data from a network client.
2. Background Art
Mainframe data processing, and more recently distributed computing, have required increasingly large amounts of data storage. This data storage is most economically provided by an array of low-cost disk drives integrated with a large semiconductor cache memory. Such cached disk arrays were originally introduced for use with IBM host computers. A channel director in the cached disk array executed channel commands received over a channel from the host computer. Moreover, the cached disk array was designed with sufficient redundancy so that data written to the cache memory would be available despite any single point of failure in the cached disk array. Therefore, most applications could consider that a write from the host to the cached disk array was completed once the data was written to the cache memory. This characteristic of the cached disk array is known as a “fast write” capability because the write operation is considered to be completed much faster than the time to write the data to disk storage.
More recently there has been a trend toward attaching storage systems to data networks so that the storage is available to multiple hosts. The hosts can range from mainframe computers to engineering workstations to commodity personal computers. Due to the “fast write” capability of the cached disk array, the data network has been seen as a limitation on the performance of the network-attached storage. There has been a continuing desire to reduce the performance penalty for attaching a cached disk array to a host through a data network instead of a dedicated channel.